The present invention relates to obtaining digital panoramic images and displaying panoramic images on computer screens. The present invention also relates to a method for displaying an initial panoramic image captured in accordance with the above-mentioned method on a screen.
FIG. 1 represents a classical device allowing a digital panoramic image to be produced and presented on a computer screen. The device comprises a digital camera 1 equipped with a fish-eye objective lens 2 having a constant field angle relative to its optical axis and preferably offering a solid image capture angle of at least 2π steradians (i.e. a field angle of at least 180°). The camera 1 is connected to a computer 5, such as a microcomputer for example, equipped with a screen 6. The connection to the microcomputer 5 may be permanent, when, for example, the camera 1 is a digital video camera, or temporary, when, for example, the camera 1 is a still digital camera equipped with an image memory.
FIG. 2 schematically represents the appearance of a panoramic image 4 projected onto a digital image sensor 3 by means of the fish-eye objective lens 2. In accordance with the most widespread industrial standard for consumer digital cameras, the image sensor 3 is rectangular in shape, to produce rectangular photographs in 4/3 format (video format). The image of the panorama projected onto the image sensor has the shape of a disk and is characteristic of the axial symmetry of fish-eye objective lenses having a constant field angle relative to their optical axis. The entire image on the rectangular image sensor therefore has dark edges that will be removed when digitally processed subsequently. This rectangular digital image comprising the image disk 4 is delivered by the camera 1 in the form of a computer file containing image points coded RGBA arranged in a two-dimensional table, “R” being the red pixel of an image point, “G” the green pixel, “B” the blue pixel, and “A” the Alpha parameter or transparency, the parameters R, G, B, A generally being coded on 8 bits.
The image file is then transferred into the microcomputer 5 which transforms the initial image disk 4 into a three-dimensional digital image, then presents the user with a sector of the three-dimensional image in a display window DW occupying all or part of the screen 6.
FIG. 3 schematically shows classical steps of transforming the two-dimensional panoramic image into a panoramic image offering a realistic perspective effect. After removing the black edges of the image, the microcomputer has a set of image points forming an image disk 10 of center O′ and axes O′U and O′V. The image points of the image disk 10 are transferred into a three-dimensional space defined by an orthogonal coordinate system of axes OXYZ, the axis OZ being perpendicular to the plane of the image disk. The transfer is performed by a mathematical function implemented by an algorithm executed by the microcomputer, and leads to obtaining a set of image points referenced in the coordinate system OXYZ. These image points are for example coded in spherical coordinates RGBA(φ,θ), φ being the latitude and θ the longitude of an image point, the angles φ and θ being coded on 4 to 8 bytes (IEEE standard). These image points form a sphere portion HS covering a solid angle of at least 2π steradians relative to the center O of the system. The microcomputer therefore has a three-dimensional virtual image one sector 11 of which, corresponding to the display window DW mentioned above, is presented on the screen (FIG. 1) considering that the observer is on the central point O of the system of axes OXYZ, which defines with the center O″ of the image sector 11, a direction OO″ called “viewing direction”.
This technique of displaying a digital panoramic image sector on a computer screen has various advantages, particularly the possibility of “exploring” the panoramic image by sliding the image sector 11 presented on the screen to the left, the right, upwards or downwards, until the limits of the panoramic image are reached. This technique also allows complete rotations to be carried out inside the image when two complementary digital images have been taken and supplied to the microcomputer, the latter thus reconstituting a complete panoramic sphere by assembling two hemispheres. Another advantage provided by presenting a panoramic image on screen is to enable the observer to make enlargements or zooms on parts of the image. The zooms are performed digitally, by shrinking the image sector displayed and expanding the distribution of the image points on the pixels of the screen.
Despite these various advantages, digital zooms have the disadvantage of being limited by the resolution of the image sensor, which is generally much lower than that of a classical photograph. Therefore, when the enlargement increases, the granulosity of the image appears as the limits of the resolution of the image sensor are being reached.
To overcome this disadvantage, it is well known to proceed with pixel interpolations so as to delay the apparition of the blocks of color which betray the limits of the resolution of the sensor. However, this method only improves the appearance of the enlarged image sector and does not in any way increase the definition. Another obvious solution is to provide an image sensor with a high resolution, higher than the resolution required to present an image sector without enlargement, so that there is a remaining margin of definition for zooms. However, this solution is expensive as the cost price of an image sensor rapidly rises with the number of pixels per unit of area. Yet another classical solution involves arranging the image sensor in a plane in which the diameter of the image disk is equal to the length of the image sensor. Thus the entire surface of the image sensor is covered but the image projected is cut off at the top and the bottom widthwise of the image sensor. The disadvantage is, in this case, a reduction in the field of view.